The present invention relates to a cathode panel for a cold cathode field emission display, a cold cathode field emission display into which the above cathode panel for a cold cathode field emission display is incorporated, and a method of producing a cathode panel for a cold cathode field emission display.
Studies of various flat panel type displays are under way as displays which are to replace currently main-stream cathode ray tubes (CRT). The flat panel type displays include a liquid crystal display (LCD), an electroluminescence display (ELD) and a plasma display panel (PDP). Further, there is also proposed a cold cathode field emission display (FED), which is capable of emitting electrons into vacuum from a solid without relying on thermal excitation, and it is of great interest from the viewpoint of a high resolution, display of high-brightness colors and a low power consumption.
A cold cathode field emission display (to be sometimes referred to as “display” hereinafter) generally has a constitution in which a cathode panel having cold cathode electron emitting portions (to be sometimes referred to as “electron emitting portions” hereinafter) disposed so as to correspond to pixels arranged in the form of a two-dimensional matrix and an anode panel having fluorescent layers which are caused to emit light by colliding with electrons emitted from the above electron emitting portions are placed to be opposed to each other through a vacuum layer. Each electron emitting portion formed on the cathode panel is constituted of one cold cathode field emission device (to be sometimes referred to as “field emission device” hereinafter) or a plurality of field emission devices.
The field emission device can be generally classified into Spindt type, edge type and flat type field emission devices.
FIG. 27 shows a conceptual view of a display to which the Spindt type field emission devices are applied, as an example, and FIG. 63 shows a partial schematic perspective exploded view of part of a cathode panel 50 and an anode panel 60 as a conventional example. The Spindt type field emission device constituting such a display comprises a cathode electrode 52 formed on a support 51, an insulating layer 53, a gate electrode 54 formed on the insulating layer 53 and a circular-cone-shaped electron emitting electrode 56 formed in an opening portion 55 penetrating through the gate electrode 54 and the insulating layer 53. A predetermined number of the electron emitting electrodes 56 are arranged in a two-dimensional matrix form, to form one electron emitting portion which constitutes one pixel. The cathode electrode 52 has a stripe form extending in a first direction, and the gate electrode 54 has a stripe form extending in a second direction different from the first direction (see FIG. 63). The cathode electrode 52 in the stripe form and the gate electrode 54 in the stripe form overlap each other, and the overlapped portion corresponds to the electron emitting portion 10. The cathode panel 50 has a plurality of such electron emitting portions.
The anode panel 60 has a structure in which a fluorescent layer 62 having a predetermined pattern (specifically, a fluorescent layer 62R to emit light in red, a fluorescent layer 62G to emit light in green and a fluorescent layer 62B to emit light in blue as shown in FIG. 63) is formed on a substrate 61, and the fluorescent layer 62 is covered with an anode electrode 63. A black matrix 64 composed of a light-absorbing material such as carbon is filled in every portion between two members of the fluorescent layers 62R, 62G and 63B to prevent the color mixing of a display image. The above order of stacking of the fluorescent layer 62 and the anode electrode 63 on the substrate 61 may be reversed. In this case, the anode electrode 63 is positioned in front of the fluorescent layer 62 when viewed from the side of a viewer of a display, so that it is required to use a transparent electrically conductive material such as ITO (indium-tin oxide) or the like, to form the anode electrode 63.
When a voltage is applied between the cathode electrode 52 and the gate electrode 54, an electric field is generated, and electrons are emitted from the top portion of the electron emitting electrode 56 due to the electric field. The electrons are attracted toward the anode electrode 63 formed in the anode panel 60 and collide with the fluorescent layer 62 which is a light emitting layer interposed between the anode electrode 63 and the transparent substrate 61. As a result, the fluorescent layer 62 is excited to emit light, and a desired image can be obtained. The operation of the field emission device is basically controlled by a voltage to be applied to the gate electrode 54.
The outline of the method of producing the Spindt type field emission device shown in FIGS. 27 and 63 will be explained below with reference to FIGS. 31A, 31B, 32A and 32B. This method is in principle a method of forming the circular-cone-shaped electron emitting electrode 56 composed of a metal material by vertical vapor deposition. That is, vaporized particles perpendicularly enter the opening portion 55. The amount of the vaporized particles which reach a bottom portion of the opening portion 55 is gradually decreased by utilizing the shielding effect of an overhanging deposit formed around an opening edge portion of the opening portion 55, so that the electron emitting electrode 56 as a circular-cone-shaped deposit is formed in a self-aligned manner. This method employs a method of pre-forming a peeling-off layer 57 on the insulating layer 53 and the gate electrode 54 for easing the removal of the unnecessary overhanging deposit, and the method will be explained with reference to FIGS. 31A, 31B, 32A and 32B.
[Step-100]
A conductive material layer composed, for example, of polysilicon for a cathode electrode is formed on a support 51 composed, for example, of a glass substrate by a plasma-enhanced CVD method. Then, the conductive material layer for a cathode electrode is patterned by a lithographic method and a dry etching method, to form the cathode electrode 52 having a stripe form. Thereafter, an insulating layer 53 composed of SiO2 is formed on the entire surface by a CVD method, and then a conductive material layer (for example, TiN layer) for a gate electrode is formed by a sputtering method. Then, the conductive material layer for a gate electrode is patterned by a lithographic method and a dry etching method, to form the stripe-shaped gate electrode 54 which is composed of the conductive material layer and has an opening portion 55. Thereafter, an opening portion 55 having a diameter, for example, of approximately 1 μm is formed in the insulating layer 53 (see FIG. 31A).
[Step-110]
As shown in FIG. 31B, a peeling-off layer 57 is formed on the gate electrode 54 and the insulating layer 53 by oblique vapor deposition of nickel (Ni) while the support 51 is turned. In this case, the incidence angle of vaporized particles relative to a normal of the support 51 is set at a sufficiently large angle (for example, an incidence angle of 65° to 85°), whereby the peeling-off layer 57 can be formed on the insulating layer 53 and the gate electrode 54 almost without depositing any nickel in the bottom portion of the opening portion 55. The peeling-off layer 57 extends from the opening edge portion of the opening portion 55 like eaves, whereby the diameter of the opening portion 55 is substantially decreased.
[Step-120]
Then, an electrically conductive material such as molybdenum (Mo) is deposited on the entire surface by vertical vapor deposition (incidence angle 3° to 10°). During the above vapor deposition, as shown in FIG. 32A, as the conductive material layer 56A having an overhanging form grows on the peeling-off layer 57, the substantial diameter of the opening portion 55 is gradually decreased, the vaporized particles which contributes to the deposition in the bottom portion of the opening portion 55 gradually comes to be limited to particles which pass the central region of the opening portion 55. As a result, a circular-cone-shaped deposit is formed on the bottom portion of the opening portion 55, and the circular-cone-shaped deposit constitutes the electron emitting electrode 56.
[Step-130]
Then, as shown in FIG. 32B, the peeling-off layer 57 is peeled off from the insulating layer 53 and the gate electrode 54 by a lift-off method, and the conductive material layer 56A above the insulating layer 53 and the gate electrode 54 is selectively removed. In this manner, a cathode panel having a plurality of the Spindt type field emission devices can be obtained.
In the field emission device, the emission of electrons from the tip portion of the electron emitting electrode 56 begins when a potential difference ΔV between a voltage applied to the gate electrode 54 and a voltage applied to the cathode electrode 52 comes to be a threshold voltage ΔVth or higher. And, for example, as the voltage applied to the gate electrode 54 increases (that is, as the potential difference Δ increases), an electron emission current caused by the emission of electrons from the tip portion of the electron emitting electrode 56 sharply increases.
Meanwhile, very clean treatment and high processing accuracy are required for producing a large-sized display. For example, for producing a color display having 380000 pixels, it is required to form 1140000 electron emitting portions. When a display is to be constituted of the Spindt type field emission devices, it is required to form tens to approximately one thousand Spindt type field emission devices per electron emitting portion. It is therefore required to form tens of millions of the fine field emission devices close to one another at intervals of several micrometers or less. For attaining a high electron emission current at a low driving voltage, preferably, the distance of the tip portion of the electron emitting electrode 56 and the opening edge portion of the gate electrode 54 is approximately 0.12 μm to 1.2 μm.
In the steps of producing the above Spindt type field emission devices, however, it is required to peel off the peeling-off layer 57 all over the large-area support (for example, glass substrate) for producing a display having a large-area screen, and the peeling of the peeling-off layer 57 causes a defect on the field emission devices. In a dry process, further, a reaction product is increasingly deposited in processing the large-area support, and the field emission devices are liable to have a defect due to particles. When an electrically conductive foreign matter (particle) is present between the gate electrode 54 and the electron emitting electrode 56, the gate electrode 54 and the electron emitting electrode 56 are short-circuited, and as a result, electrons are no longer emitted from the field emission device, so that a black spot appears on a display screen. In the cathode panel, generally, there are arranged a plurality of columns of cold cathode electron emitting regions each of which has a plurality of electron emitting portions arranged one-dimensionally (in the form of a stripe). When a short circuit is formed in the field emission device, there are some cases where the whole of one column of the stripe-shaped electron emitting portions including the short-circuited field emission device can no longer perform complete displaying.
In one method of avoiding the problem caused by the above short circuit of the field emission device, for example, a resistance layer having a resistance of about 2 MΩ is formed between the electron emitting electrode 56 and the cathode electrode 52, as is schematically shown in FIG. 64A. When a field emission device is in a short-circuited state, a leak current consequently flows between the gate electrode 54 and the cathode electrode 52 through the electron emitting electrode 56 and the resistance layer, which leads to an increase in consumption power. When a difference between a voltage applied to the gate electrode 54 and a voltage applied to the cathode electrode 52 is ΔV, and when the resistance layer has a resistance value R, the consumption power P consumed due to short circuiting of the field emission devices can be calculated by the following equation, in which n is the number of field emission devices in a short-circuited state.P=n(ΔV2/R)
The consumption power P can be decreased by increasing the resistance value R of the resistance layer. When the resistance value R is increased, however, there is brought a state where the resistance value R is added to an inter-layer capacitance component C between the gate electrode 54 and the cathode electrode 52 as is shown in FIG. 64B, and, as a result, a time constant increases. Driving signals to be applied to the gate electrode 54 and the cathode electrode 52 are therefore delayed, and as a result, the operation speed of the display is deferred. Therefore, the resistance value R of the resistance layer cannot be increased much, and if possible, it is preferred not to provide the resistance layer.
Another problem with the field emission devices is that the field emission devices vary in electron emitting characteristics. The field emission devices are formed on the cathode panel in the number of hundreds of thousands to hundreds of millions by the same process and even if these field emission devices look identical when observed through an electron microscope, the field emission devices vary in threshold voltage ΔVth. When a field emission device has an extremely low value of the threshold voltage ΔVth, it comes to be in an operation state even in a potential difference state where field emission devices which have a normal threshold voltage ΔVth do not operate. As a result, there is caused a problem that a bright spot appears on a display screen.